Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer an important improvement over lead-acid batteries. Typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and can cause serious environmental problems during manufacturing and disposal. The useful lifetime of flowing electrolyte batteries is, on the other hand, not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
FIG. 1 illustrates a basic zinc-bromine flowing electrolyte battery 100, according to the prior art. The zinc-bromine battery 100 includes a negative electrolyte circulation path 105 and an independent positive electrolyte circulation path 110. The negative electrolyte circulation path 105 contains zinc ions as an active chemical, and the positive electrolyte circulation path 110 contains bromine ions as an active chemical. The zinc-bromine battery 100 also comprises a negative electrolyte pump 115, a positive electrolyte pump 120, a negative zinc electrolyte (anolyte) tank 125, and a positive bromine electrolyte (catholyte) tank 130.
To obtain high voltage, the zinc-bromine battery 100 further comprises a stack of cells 135 connected in a bipolar arrangement, the stack of cells 135 producing a total voltage higher than that of the individual cells.
For example, a cell 135 comprises half cells 140, 145 including a bipolar electrode plate 155 and a micro porous separator plate 165. The micro porous separator plate 165 often includes a plurality of spacing ribs, which separate surfaces of the bipolar electrode plate 155 and the micro porous separator plate 165. Without such spacing ribs, the micro porous separator plate 165 could move too close to the electrode plate 155 and restrict flow of electrolyte.
The zinc-bromine battery 100 then has a positive polarity end at a collector electrode plate 160, and a negative polarity end at another collector electrode plate 150.
A chemical reaction in a positive half cell, such as the half cell 145, during charging can be described according to the following equation:2Br−→Br2+2e−  Eq. 1
Bromine is thus formed in half cells in hydraulic communication with the positive electrolyte circulation path 110 and is then stored in the positive bromine electrolyte tank 130. A chemical reaction in a negative half cell, such as the half cell 140, during charging can be described according to the following equation:Zn2++2e−→Zn  Eq. 2
A metallic zinc layer 170 is thus formed on the collector electrode plate 150 in contact with the negative electrolyte circulation path 105. Chemical reactions in the half cells 140, 145 during discharging are then the reverse of Eq. 1 and Eq. 2.
However, a problem with the zinc-bromine flowing electrolyte battery 100 of the prior art is that the spacing ribs reduce an effective surface area of the electrode plate 155, and in some cases may obstruct more than ten percent of the flow surface of the electrode plate 155. This in turn reduces an efficiency of the zinc-bromine flowing electrolyte battery 100.
A further problem with the basic zinc-bromine flowing electrolyte battery 100 of the prior art is that dendrites can form on a surface of the electrode plate 155, which in turn can puncture the micro porous separator plate 165. A puncture of a micro porous separator plate 165 can enable the mixing of positive and negative electrolyte, thus chemically discharging the battery, or more critically causing an electrical short circuit. Furthermore, it is impractical to replace a micro porous separator plate 165 that has been punctured.
Yet a further problem with the basic zinc-bromine flowing electrolyte battery 100 of the prior art is that zinc plate de-lamination can occur. In such case, plated zinc separates from a surface of the electrode plate 155, which can in turn damage the zinc-bromine flowing electrolyte battery 100. This is particularly problematic when large temperature changes occur at a high state of charge.
There is therefore a need to overcome or alleviate many of the above discussed problems associated with flowing electrolyte batteries of the prior art.